Magician's Gambit - David Eddings (Book 3 of the Belgariad)

Travelling through ever more dangerous realms, Garion and his companions pursue the stolen Orb. Among them Ce’Nedra, the Imperial Princess – one whose fate seems bound to his. As they get closer to their goal and battle looms, Garion’s powers of sorcery – and his spirit – are tested to their limits.